Replace it with a Seagate and you shouldn't have any problem. (Manufacturers don't choose WD because they're good, they choose them because they get them cheap.)
Are you testing the drives before installing them? I test drives before they get fitted, seen quite a few DOA drives in the past.
Ask the customer how they handle the machine, is it kept on a desk 24/7 or do they take it to work/school?
I'm voting shock damage on this if it is happening a month or so down the line though.
+1 for checking how the customer uses the laptop.
We had a guy with the same issue, after the 2nd replacement we found out he had it mounted in his 4x4 truck and was always on while he drove around his farm..
People really have no idea of the microscopic scale of the gap between the surface of the platters and the read/write head of a hard drive, separated only aerodynamically in the boundary layer flow. Their eyes might glaze over with a technical description, but the following analogy from Scott Mueller's Upgrading and Repairing PCs, 15th edition shows that, even magnified over 300,000x, it's still tiny. I explain this to clients with laptops, so they get some idea of how small a shock it takes for the head to bridge that gap and damage the surface of the platters.
Ultimate Hard Drive Analogy
Really? What a full long test for every fitting?